A conventional technique for viral inactivation of plasma uses a colouring substance as a virucidal substance, for example methylene blue or one of its derivatives.
The principle of this technique is based on photochemical reactions between the colouring substance and the viral DNA or RNA which may be present in the biological fluid.
Exposure of the colouring substance to light brings about a photochemical reaction which transmits energy to the DNA and RNA molecules so that the virus is inactivated.
During these photochemical reactions, the colouring substance is not removed so that it remains in the biological fluid after exposure to light.
After the use of this viral inactivation technique, a very small amount of the colouring substance may be left in the biological fluid and thus be transfused into the patient at the same time as the biological fluid.
However, recent studies seem to show the possible toxicity of certain colouring substances used, and in particular methylene blue, when they are injected into the patient.
So much so that many countries are demanding the systematic removal of colouring substances prior to injection of the biological fluid into the patient.